Chemistry Reference
In-Depth Information
biopolymers, this crosslinking reaction is allowed by the mobility of the
biopolymer chains and the reactivity of phenolic compounds. The main
consequence is the transformation of hemicelluloses, the most hydrophilic
polymers, into a network with a more or less hydrophobic character. The
amount of crosslinking is low, but the improvement of the physical properties
of the wooden material is significant.
67
In the case of protein films,
58,68
the treatment is carried out at lower
temperatures (95-125 1C) than those used in the present study (200 1C),
allowing the denaturation and the coagulation of proteins and the formation of
new interactions (hydrophobic and cysteine bridges) in the loose network of
films obtained by casting.
When applied to the ESTOC, the thermal treatment should have an influence
only on fibres, because the protein network is well established. However, when
phenolic compounds interact with proteins, they may form new bonds at higher
temperatures, even in an inert atmosphere.
The first consequence of the thermal treatment is a loss of mass, which
reaches 6.4% of the initial mass after equilibration at 25 1C and 60% R.H., and
is associated with a decrease of volume. Finally, the apparent density decreases
by about 0.1 during the first minutes of the treatment (Table 7.7). At the same
time, the evaporation of water at this temperature results in irreversible
modification of the treated samples.
Overall, thermal treatment leads to a decrease of the mechanical resistance of
the specimens; the stress at break in bending and in tension decreases by 30%
(Table 7.7), with an increase of their rigidity in the axis perpendicular to fibres,
while the bending modulus increases during the treatment. The same consequences
were observed during the treatment of wood.
66
This is related to the replacement
of water plasticising molecules by interactions between polymeric chains.
The mechanical resistance loss (Table 7.7) is stabilised for a cooking duration
higher than 3 min/g. For shorter times, the treatment involves first the disap-
pearance of a part of the equilibrium absorbed water: at 1.25 min/g, the
apparent density after equilibrium at 25 1C and 60% R.H. has already fallen to
1.2 and the mechanical resistance decreased greatly (Table 7.5). On the other
hand, the kinetics of water absorption was practically unmodified (Figure 7.14),
indicating that the treatment duration was not sucient to form new covalent
d
n
9
r
3
n
g
|
5
d
y
y
f
n
n
3
.
Table 7.7 Mechanical properties in tension (s
t
: stress at break, E
y
: Young
modulus) and bending (s
f
: stress at break, E
f
: bending modulus) of
thermally treated ESTOC samples.
57
Time (min/g)
0
1.25
3.75
7.5
15
Density
1.34
0.03
1.21
0.04
1.19
0.06
1.20
0.05
1.16
0.04
s
f
(MPa)
37
3
34
4
24
4
27
5
25
2
E
f
(GPa)
3.3
0.3
4.4
0.6
3.8
0.6
4.4
0.2
4.2
0.3
Time (min/g)
0
0.55
1.67
3.33
6.67
s
t
(MPa)
12.5
2.7
11.5
2.8
8.7
2.6
7.8
1.1
7.4
1.8
E
y
(GPa)
2.0
0.1
2.1
0.3
1.7
0.3
1.9
0.3
1.9
0.5
Search WWH ::
Custom Search